Reconfigurable networks that employ active switching, add-drop multiplexing, and wavelength routing cause power level variations. Power levels may also vary as a result of component degradation due to aging or reduction in fiber coupling efficiency. To maintain the power level at the receiver at its optimum value, electrically controllable attenuators may be used. Attenuation of the input signal is also used to extend the dynamic range of the receiver by reducing the input power below the saturation level.
Attenuation of an input signal is accomplished by using an optical attenuator that is typically placed before the receiver. A Variable Optical Attenuator (VOA) is a preferred type of attenuator due to its ability to continuously adjust the amount of optical attenuation.
Based on their principle of operation, VOAs can be divided into different groups.
The simplest type of attenuator is operated by causing relative, variable lateral offset of the input and output fibers, so that the overlap between the input light beam and the core of the output fiber can be adjusted. This type of attenuator is known to be extremely sensitive to small changes in the relative position of the fibers due to, for example, environmental changes (temperature, vibration, etc.)
Several other VOA types achieve attenuation by miscoupling the incoming light while using collimating and/or focusing optics to transform the incoming light. A VOA with a pair of parallel mirrors placed in the optical path of a collimated beam is disclosed in U.S. Pat. No. 6,149,278. Attenuation in such is achieved by rotating the mirrors so that the field distribution in the focal plane of the focusing lens is miscoupled relative to the core of the output fiber. In another U.S. Pat. No. 6,137,941, therein is described a VOA with a pivoting micro-mirror. Rotation of the mirror changes coupling to the output waveguide by lateral displacement (miscoupling) of the light distribution relative to the waveguide which thus causes attenuation of the signal.
The other group of attenuation types uses two lenses in the optical path, where the first lens delivers the incoming light either collimated or focused to the attenuation mechanism. The attenuated balance of the signal is thereafter directed to a focusing lens for coupling to an output fiber. Specifically, these types of attenuators use light absorbers (neutral density filters in U.S. Pat. Nos. 6,292,616; 6,130,984; 4,904,044; 4,591,231; and a bulk absorber in U.S. Pat. No. 5,325,459), or mechanical blockers (U.S. Pat. Nos. 6,275,320; 6,163,643; 5,745,634; 5,087,122) as mechanisms for optical attenuation. In particular, this group conventionally employs a pair of lenses so that the light absorber or blocker which is positioned between these lenses encounters collimated (U.S. Pat. Nos. 6,292,616; 6,130,984;) or focused light (U.S. Pat. Nos. 6,275,320; 6,163,643).
Combining the aforementioned VOAs with a receiver requires coupling of the output fiber to the receiver, thus causing additional insertion loss and, as a consequence of that, degradation in the sensitivity of the combined solution. Integration of that type of VOA with receiver in a single package therefore requires accurate alignment of the active area of the detector to the output beam of VOA, thereby increasing complexity and cost of the packaged solution. In addition to that, the above VOA solutions (except the version with direct fiber coupling) employ two collimating and/or focusing lenses in the optical path. Reduction in the number of the optical components is desirable to further reduce the insertion loss, size, and cost of the integrated VOA and receiver solution.
It would be desirable to provide a receiver with extended dynamic range that integrates attenuation and coupling functionality (e.g. integrates VOA and a Photodetector (PD) in a single packaged device) with improved sensitivity, as well as reduced cost and size.
Turning now to a further aspect of the subject invention, it may be seen that with the growth of optical communication systems and the continuous rising demand for network capacity, an increasing demand exists for high-speed photodetectors (PDs), e.g. p-i-n detectors.
The two principal bandwidth limits of high-speed photodetectors, such as p-i-n PDs, are the bandwidth associated with the carrier transit-time and the bandwidth associated with RC time-constants (see, for example, Ref. 1). The carrier transit-time, photodetector capacitance, and quantum efficiency are interdependent, although, if one reduces the thickness of the absorption layer and the photosensitive area of the photodetector, then the transit-time can be reduced and the bandwidth may be increased. However, reducing the absorption layer thickness will also increase the capacitance of the photodiode which tends to decrease bandwidth. The capacitance can be independently reduced by making the device area smaller, but reduction in photosensitive area makes the efficient coupling of light difficult thus reducing the coupling efficiency and increasing the sensitivity to optical misalignment. The fundamental limitation on the coupling efficiency is due to diffraction when free-space micro-optical components are used.
In all cases, the reduction in absorption layer thickness lowers the intrinsic quantum efficiency of the detector for surface-normal illumination. Several approaches have been developed to circumvent the loss of quantum efficiency (QE) using thin absorption layers.
Edge-coupled waveguide (WG) PDs have been developed and can achieve high speed and high QE if the modal coupling efficiency is high, but coupling tolerances are very small, especially in the direction normal to WG surface. Efficient coupling to WG PDs also requires special means for transforming the mode size from an input fiber to that matching the mode of the WG photodiode. These mode transformers introduce additional sources of losses to the device and are difficult to fabricate.
Alternative coupling approaches are based on the angular (off-normal) incidence to PD photosensitive area, and have been demonstrated using refraction on an angled facet on the photodiode or total internal reflection (TIR). There are several disadvantages in using these existing techniques. The refractive facet or V-groove for TIR mirror is difficult to fabricate, as well as to control the surface geometry with high accuracy. In case of the etched TIR mirror, the etching weakens the (already) fragile PD chip. In addition, the mirror surface should be positioned accurately relative to photosensitive area of the PD, which may be difficult to accomplish. The above coupling schemes also suffer from poor coupling efficiency due to beam divergence caused by diffraction.
It is, therefore, desirable to provide an edge coupling subassembly that does not require the use of PDs with the above mentioned deficiencies, does not require fabrication of special features on the PDs such as mode spot size converters, refractive facets or TIR mirror V-groves, and at the same time, allows achievements in increased responsivity, high bandwidth, and is well suited for high-yield high volume fabrication.
In view of the foregoing, it apparent that there exists in the art a need for apparatus or method which overcomes, mitigates, or solves the above problems in the art. It is a purpose of this invention to fulfill this and other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.